1. Field of the Invention
This invention relates generally to systems for the reproduction of recorded information, such as video information recorded on a disc and, more particularly, to systems for centering a light beam on an information track on such a disc.
Video and other types of information can be stored on a disc as a succession of light reflective and light non-reflective regions along a spiral information track on the surface of a disc-shaped record carrier. In reproducing the video information a video disc player employs an optical system for directing a radiant reading beam, such as a laser beam, onto the information track, and for detecting a reflected beam indicative of the reflectivity of the disc surface scanned by the laser beam as the disc is rotated. In a disc player of the type with which the invention may be employed, the intensity of the reflected beam takes the form of a high frequency carrier signal which is frequency modulated by the composite video information.
In order to store a reasonably large amount of information on the disc, successive turns of the information track must be so closely spaced that even a slight degree of disc eccentricity would cause the beam to traverse a number of adjacent tracks on each revolution. Consequently, some means must be provided for applying transverse or radial corrections to the beam position, so that it substantially follows along the center of the information track.
2. Description of the Prior Art
In a commonly assigned copending application, Ser. No. 963,196, filed Nov. 24, 1978, entitled "Dithered Center Tracking System", filed in the name of Lawrence S. Canino U.S. Pat. No. 4,232,201 a dither tracking system is disclosed, wherein a constant frequency dither signal is applied to oscillate the reading beam, in a radial sense, as it moves along the information track. A bandpass filter is provided for deriving an intermediate signal indicative of the effect of the dither on the detected signal derived from the information track.
Switching circuitry derives an error signal from the intermediate signal and the polarity of the dither signal. The switching circuitry includes an inverter, and a switch for selecting either the intermediate signal or the inverted intermediate signal, in accordance with the polarity of the dither signal. Although this technique has proved satisfactory for many applications, it provides only one beam steering device for both application of the dither signal and tracking of the information track. This precipitates unwanted resonances at the lower tracking frequencies. At such high storage densities these unwanted resonances can cause an inaccurate error signal to be generated. Accordingly, there has been a need for an improved and more reliable technique of dither tracking which avoids the use of a single beam steering device. The present invention is directed principally to this end.